The use of crosslinking reactions for membrane modification to impart desirable properties on the membrane has been the subject of numerous inventions.
In U.S. Pat. No. 3,232,916 a method for crosslinking poly(vinyl-alcohol) membranes by heat treating is described. The crosslinked membranes are useful as partitions in batteries and electrolytic cells.
U.S. Pat. No. 3,265,536 describes crosslinking poly(vinyl-alcohol) using formaldehyde. The resulting membrane is useful in fuel cell applications.
Aromatic polyamide membranes were cross linked by chemical reaction with aldehydes, polyamines, peroxides, or by irradiation. The resulting membranes were described in U.S. Pat. No. 3,904,519 as having improved flux stability.
U.S. Pat. No. 3,951,621 teaches that a nylonpoly(vinyl-alcohol) membrane was crosslinked by incorporating dicarboxylic acids into the casting dope and subsequent heat treatment. These membranes, after saturation with 6M silver nitrate solution, gave good paraffin/olefin separations.
U.S. Pat. No. 4,020,142 describes a method for improving the compaction resistance of a polybenzimidazole membrane by crosslinking the membrane with strong polybasic acids.
The water-salt separation capability of polyamide and polyhydrazide membranes was improved by crosslinking the membranes with metal ions of the alkali earth metal, transition metal, aluminum or lead groups, as described in U.S. Pat. No. 4,086,215.
The use of plasma to effect membrane crosslinking was described in U.S. Pat. No. 4,046,843 and U.S. Pat. No. 4,163,725. The resulting membranes were described as having good separation characteristics in water-salt separations.
See also U.S. Pat. No. 3,140,256; U.S. Pat. No. 3,864,289; U.S. Pat. No. 3,585,126; U.S. Pat. No. 3,669,954; U.S. Pat. No. 3,837,900; U.S. Pat. No. 4,186,238; U.S. Pat. No. 4,194,024; U.S. Pat. No. 4,200,558; U.S. Pat. No. 4,175,183; Japanese patent application No. 038137.
U.S. Pat. No. 3,556,991 describes a method for the recovery of aromatic hydrocarbons using the steps of solvent extraction, dialysis and fractionation of the extract. In the dialysis all the extracted aromatic component is separated from the extraction solvent by dialysis of the aromatic component through the membrane. Examples of suitable membrane materials are listed at column 4, line 16ff of the patent and include cellulose esters, cellulose ethers, blends of cellulose esters and cellulose ethers and other cellulose derivatives. Other suitable membranes include films of 2-chlorobutadiene, polyethylene, polypropylene, polytetrafluoroethylene, copolymers of butadiene and styrene, copolymers of isoprene and isobutylene, vinyl chloride, vinylidene chloride, crosslinked copolymers of ethylene and propylene, vulcanized natural rubber and the like. This process may also employ cellulose ester, ether and other cellulose derivative membranes modified by reacting the free hydroxyl groups of such members with organic reagents such as aldehydes, aldehyde-alcohol mixtures, organic dissocyanates, organic monoisocyanates, etc., or by aldehyde followed by an organic diisocyanate.
In all cases cited, the crosslinking reaction was used to impact beneficial properties to the membrane, e.g., compact resistance, salt rejection. It is to be noted that nowhere in the literature was there mentioned the use of crosslinking reactions to reduce RC membrane hydrophilicity and improve RC membrane separation efficiency in the separation of organic liquids.
The reaction of crosslinking chemicals with the hydroxy groups present in the anhydroglucose units of cellulose has been investigated quite extensively, especially in the field of textile finishing (see for example Kirk-Othmer's Encyclopedia of Chemical Technology, Vol. 22, 3rd Edition, p. 770-790, 1983 Wiley N.Y.). However, such reactions have not been investigated for use to improve the performance as well as to reduce the hydrophilicity of RC membranes for use in non-aqueous separations.